Prior art paint booths are well known. A typical prior art paint booth for painting an exterior surface of a vehicle body in a continuous conveyance and stop station system includes an enclosure housing and a plurality of paint applicators. In one configuration, the applicators are mounted on an inverted U-shaped support structure that includes two vertical supports, one on either side of the path of travel of the vehicle body, and connected at a top thereof by a horizontal support structure. The support structure facilitates painting of a top surface of the vehicle body, and the horizontal beam can be fixed or have an additional degree of freedom to move along the top surface of the vehicle body being painted. Another painting device is used in the same painting zone to paint sides of the vehicle body, and generally is not capable of moving laterally along the length of the vehicle body. Disadvantages of this type of painting apparatus include lack of flexibility to provide optimized standoff distance between the vehicle body surface and the applicator, and inefficient use of the allotted painting cycle time. The paint applicators of the painting devices adapted to paint the top surface of the vehicle are mounted on a common beam. Therefore, the distance between each paint applicator and the surface to be painted varies with the contours of the vehicle body. The paint applicators of the painting devices adapted to paint the sides of the vehicle include applicators that do not move transverse to the path of the vehicle body. Accordingly, the paint applicators can only paint a portion of the vehicle body that is in front of the applicator, leaving a substantial portion of the available cycle time unused.
A more recent alternative to the support structure is a floor-mounted robot disposed along the sides of the paint booth. The robots include spray guns or rotary applicators (bell machines) mounted thereon for directing atomized paint toward the vehicle body. While rotary applicators have advantages over spray guns, there are some associated disadvantages. The prior art floor mounted robot, especially robots having rotary applicators, are costly and limit visual access to the spray booth. The bell machines require more bells for the same throughput due to limited orientation capability. The additional bells use more paint per vehicle due to the waste generated by each bell during a paint color changing operation. Prior art floor mounted robots also require significant booth modification when installed in existing paint booths, thereby increasing installation time and cost, and requiring more floor space within the paint booth. The rail axis of floor mounted robot requires doors at both ends of the paint booth. The waist axis of the floor mounted robot requires an additional safety zone at the ends of the spray booth, and the rail cabinets of the floor mounted robot encroach into aisle space. The floor mounted robot also requires frequent cleaning due to a down draft of paint overspray causing paint accumulation on the robot arm and base, which results in higher maintenance and cleaning costs.
Due to the conductivity of the waterborne paint, it is necessary to electrically isolate the grounded bulk paint supply system from a charged local dispensing canister and spray application system. In the prior art, the bell applicator, canister, canister drive, electrostatic cascade, and docking interface were all integrated into a single unit mounted on the robot wrist as shown in U.S. Pat. Nos. 5,293,911 and 5,367,944. Such an applicator has the following shortcomings: 1) the applicator is heavy, expensive, and subject to damage via collision with objects in the painting booth; 2) the applicator docking with a docking station must occur in a fixed booth position which limits process flexibility; 3) the docking process takes cycle time as the robot must travel to and from the docked position, and the canister filling cannot start until the applicator reaches the docked position; and 4) the docking hardware is expensive and unique to waterborne systems.
To prepare the robot for a painting operation, the canister must be filled with paint. To fill the canister with paint, a piston slidably disposed in the canister is drawn away from the cylinder bottom and an applicator valve is opened, thereby introducing a small amount of air into the canister. The paint is then caused to flow from a selected color valve, through an isolation line, and into the canister. As the initial volume of the canister is filled through a trigger passage of the applicator, air is pushed out of the system through the applicator until the paint reaches a restriction in the trigger passage. The restriction causes an increase in the fluid pressure in the canister due to the viscosity difference between the paint and the air being displaced by the paint. The pressure increase causes a torque applied by a drive motor to increase, which can be sensed and used to adjust the rate of filling of the canister. Once the canister and applicator are filled, air in the canister is removed. To remove the air from the canister, an amount of air and paint is expelled from the canister through the applicator until the air is removed, thereby wasting the amount of paint expelled. Another filling operation known as the pressure based fill through injector tip mode of filling the canister utilizes the torque feedback to determine when the paint will fill the canister. A single torque feedback value is typically used for the filling operation of each of the colors. However, because the viscosities and bulk pressure of the paints vary from color to color, time based filling operations may lead to wasted paint (time too long) or an improperly filled system (time too short).
The piston may be utilized to optimize the canister fill operation time. First, if the fill rate of paint into the canister is known or can be automatically measured, the rate at which the canister piston mechanism is drawn away from the canister bottom may be adjusted to minimize the pressure drop of the incoming paint, and decrease the fill time. The fill rate may be sensed by measuring either servo error (positive or negative) or motor torque feedback applied to the piston. Second, the piston may be drawn away from the canister bottom at a rate known to be slightly below the system fill rate. However, as the paint rapidly fills the canister, air may become entrapped in the canister and mixed with the paint.
The grounded bulk paint supply must be isolated from charged system components to militate against voltage leakage and electrostatic erosion. A method to isolate the bulk paint supply system from the charged paint dispensing canister is to clean and dry the paint transfer line between the supply system and the canister. In an automotive-type painting system (rapid color changing on a continuous conveyance type system), a dump line is typically connected to and in fluid communication with the bell applicator or other portion of the system downstream from the canister. When cleaning the interior of the canister, the piston is drawn away from the canister bottom. The piston is cycled in and out of the canister as a solvent and air mixture is introduced into the canister to facilitate effective cleaning of the area between the piston and the bottom of the canister. Simultaneous to the cleaning of the canister with a solvent and air mixture, a paint line from the canister to the applicator is backflushed. As the piston cycles and is caused to slidably enter the canister toward the applicator, the solvent and air mixture is forced out of the canister and through the dump line. After canister cleaning, the system is ready to be filled with a different color of paint.
This method of cleaning the robot has numerous shortcomings, including: 1) a time to clean and dry the line and provide high voltage isolation exceeds the allotted dwell time between the vehicle bodies being painted; 2) paint residue remaining on the walls of the transfer line, the dump line, or the interior of the canister may lead to a high voltage leakage causing electrostatic erosion that may burn holes in the transfer line, the dispensing system, the supply line to the applicator, or the waste collection lines; 3) an amount of waste that is left in the paint transfer line is excessive when compared to other means of isolation; and 4) because the solvent and air mixture containing paint residues is caused to flow through the dump line downstream from the solvent and air mixture input, paint residue may remain at the connection between the dump line and the canister.
As environmentally friendly waterborne coatings become more popular, customers are demanding reductions in the time and material waste associated with preparing the automatic system for electrostatic painting. The paint fluid delivery system is a key component in the application of waterborne coatings. A direct charge waterborne fluid delivery system is required to accomplish the following: clean the application system and prepare it for loading the next coating material; load the desired coating material from the bulk supply system (paint circulation system); electrically isolate the loaded quantity of paint from the grounded bulk supply system; and precisely control the rate of flow (metered dispense) from the delivery system to the coating applicator.
For example, when painting car bodies in automotive final assembly paint shops it is common to change colors often. Typical color batch size for body painting is a group of 1-5 cars. Color change time ranges between 6-15 seconds or 10-25% of the available cycle time per car. The amount of paint wasted per robot in the color change process is typically between 12-50 ml or 5-10% of the paint used by a particular robot. Low color change and refill waste are important design factors for automotive final color change systems. Refill and color change time are also important.
As a further example, when painting plastic add-on parts such as fascia, body side claddings, or instrument panels in automotive component painting lines, batch sizes are larger and color changing is less frequent; however, the cycle time per part is also less. Parts are painted in batches of 10 to 200 parts and it is desired to paint parts continuously or without dwell time between parts. In this type of painting system it is typical to leave a gap between batches of parts for color change.
Simplicity of design is important to the reliability of the system. For example, key fluid delivery design elements of a direct charge coating system include:                1. cycle time to refill the same color;        2. paint and cleaning solvent waste when refilling the same color;        3. cycle time to change to a new color;        4. paint and cleaning solvent waste when changing to a new color;        5. flow rate demand on paint circulation system;        6. equipment cost; and        7. system complexity and reliability;        
The industry currently lacks a cost effective and reliable direct charge fluid delivery system that is capable of providing the benefits of fast color change and fast refill needed for automotive body and component painting systems.
Today's voltage block systems are mainly single canister systems. The single reservoir system with single voltage block is simple, reliable, and wastes little paint, but the color change and refill time is excessive. The single canister must be filled quickly, which also puts high demand on the paint circulation system. Color change and refill can be executed in 8 to 15 seconds when 0-4 seconds is desired.
Parallel fluid circuits for solvent based paints, also called dual purge systems, have been used in the past to reduce color change time. The parallel systems generally have multiple flow control and flushing systems. While one side is painting the other side is getting the next color ready. The parallel circuits are designed for solvent based paints having significantly lower conductivity and cannot be used for waterborne applications. The painting side is charged and therefore requires the next color loading side to be isolated from the painting side.
Most of the prior art systems are extremely complex. Having many valve and voltage blocking devices and moving parts in contact with paint, these systems are difficult to maintain and operate.
It would be desirable to provide a robotic painting system and a method of operating the painting system, wherein a color change time and a paint waste are minimized and a cleaning operation of the system is optimized.